The present invention relates to a method and
us for detection of leakage of refrigerants from equipment such as freezers, refrigerators, air conditioners, dehumidifiers and like equipment having compressor-evaporator-condenser circuits containing a refrigerant as the working fluid. Usually, the refrigerant is a halogenated hydrocarbon.
It is desirable during the course of manufacture of such equipment to test for leakage of the refrigerant from the compressor-evaporator-condenser circuit, since, firstly, leakage of the refrigerant affects the durability and service life of the equipment, and, secondly, many commonly used refrigerants are considered to pollute the atmosphere and to play a part in the depletion of the earth's ozone layer.
Halogen sniffers have been used to test for leakages. Halogen sniffers, however, have quite limited sensitivity and under usual operating conditions are not capable of detecting very small rates of refrigerant leakage, of less than about 1/2 oz per year (9.times.10.sup.-5 std. atm cc/sec) FREON-12 (trade-mark), for example. Moreover, halogen sniffers are usually sensitive to smoke and solvent vapours and, in addition, the ambient air in the manufacturing plant often contains substantial concentrations of halogenated hydrocarbon chemically different from the refrigerant compound, since halogenated hydrocarbons are commonly used as blowing agents in the foamed insulation present in refrigerators, freezers and the like. The halogen sniffers are incapable of distinguishing between different halogenated compounds and are therefore not effective in detecting a leak of one halogenated hydrocarbon in an ambient atmosphere containing another halogenated hydrocarbon. To avoid this problem, refrigerators, freezers, etc. have been warehoused for approximately one week after manufacture in order to allow out-gassing of blowing agents from the foamed insulation before subjecting the equipment to leakage testing using halogen sniffers. The costs of storage and the need to maintain a large inventory of finished equipment before it can be shipped have added significantly to the costs of manufacturing of the equipment.
U.S. Pat. No. 3,578,758 (Altshuler) and U.S. Pat. No. 4,785,666 (Berquist) disclose methods for the detection of leaks from electronic components such as encapsulated semiconductor devices using a tracer gas, normally helium, using a mass spectrometer. As is well understood by those skilled in the art, a mass spectrometer ordinarily comprises an elongated evacuated detector chamber having at one end an ionizer at-which the molecules of a sample introduced into the detector chamber are ionized. Several methods of ionization are known to those skilled in the art. Ordinarily, ionization is accompanied by fragmentation of the molecules. At an opposite end of the chamber an ion detector is located. Means are provided for accelerating the ions (of whole molecules or of molecular fragments) from the ionizer toward the detector, and mass analyzer means are provided for separating ions from the path between the ionizer and the detector selectively according to mass, so that ions falling within a predetermined mass range are incident on the ion detector. In this manner, a signal can be obtained indicative of the abundance of ions or ion fragments of selected mass within the detector chamber. The greater the resolution of the spectrometer the narrower the mass range may be. Again various ion acceleration means, mass analyzer means and detection means are known to those skilled in the art. It is also known that in order to achieve the greatest sensitivity, an ion detector may comprise an electron multiplier which amplifies the current resulting from the impingement of the ions.
In the past, the composition of the gases analyzed in mass spectrometers has been carefully controlled. For example, in the Altshuler U.S. Pat. No. 3,578,758, an electronic component to be tested for leakage is first impregnated with tracer gas such as helium, and care is taken to exclude moisture and other contaminants by placing the impregnated test component in a sealed test container which is evacuated and purged with an inert dry purging gas such as nitrogen before the detector chamber of the mass spectrometer is connected to it. The mass spectrometer is adjusted to be responsive to helium and non-responsive to nitrogen, so that a signal from the mass spectrometer indicates leakage of helium from the component under test. In the Berquist U.S. Pat. No. 4,785,666 the procedure differs in that the sealed test container is purged with nitrogen at atmospheric pressure and a needle valve probe may be used to sample the gas in the test container adjacent the container undergoing test. In Berquist, the test container is preferably less than 5 ccs in volume. As will be appreciated, it would be impracticably expensive to charge large pieces of equipment such as refrigeration equipment, which have a large capacity of working fluid or refrigerant, with helium or other inert gas and it would be impracticable to maintain such equipment, which may be five or six feet in its longest external dimension, in a test container purged with nitrogen or other inert gas during a test procedure.
Moreover, it would be expected that a sensitive mass spectrometer instrument employing an electron multiplier could not be used to directly sample the ambient air atmosphere adjacent refrigeration equipment within a manufacturing plant. Such atmosphere contains moisture, hydrocarbon solvents and hydrocarbon sources such as the refrigerants and blowing agents referred to above. It would be expected that moisture would impair the function of the active components of the spectrometer within the detector chamber. The gain of the electron multiplier is known to be strongly dependent on the condition of its active surfaces and is susceptible to hydrocarbon contamination. Moreover, in the presence of large numbers of different molecular species within the detector chamber it would be expected to be difficult to resolve a selected mass peak. As a result, at any selected mass range, large detector currents would be expected to occur. At high gain levels, excessive electron multiplier output currents would be expected to cause saturation resulting in loss of multiplier sensitivity.